*4.5. Coatings with the Addition of Two or More Elements*

Quarternary and higher-order systems have been explored to further optimise coating properties. However, this has also been pursued from an industrial perspective dealing with patent issues to obtain unique selling points (USPs) or to supersede patents from competitors. However, the complexity quickly increases, and there is the potential for cross-reactions between the alloying elements. Even if the results achieved do not generally outperform the simpler coatings, a study of that research will reveal indicators of potential directions for coating development. A detailed discussion is beyond the scope of this paper; however, selected publications on Al-Cr based multinary coatings are listed below.

Si-Mo sputtered [142], Si-Ni sputtered [143,144], Si-Ti arc [145,146], Si-W arc [78,135], Si-W sputtered [136], Si-Ni sputtered [137], Si-C sputtered [133,147], Si-B sputtered [134], Si-Mg [148], Ti-C sputtered [114], MoCu sputtered [106], Si-Y-O sputtered [138], Ti-Si-Y arc [149], and Nb-Ti-Si sputtered [150].

In addition, even high-entropy coating modifications are the focus of R&D activities [150,151].
